Motors used in the automobiles and various kinds of industrial machines arc operated under various circumstances, for example, in a cold district or a high-temperature engine room. Therefore, bearings used for the motors are required to be practicable across a wide temperature range over a prolonged duration of time. Improvement of the motor efficiency has also been desired along with the recent increased environmental consciousness, so that the bearings used for the motor are strongly required to satisfy the low torque performance.
In addition, the bearing performance largely depends on the performance of grease enclosed in the bearing. In light of this, special attention should be paid to choice of grease. For example, when it is cold, the increased viscosity of the base oil used in the grease may readily produce the problem of uneven oil film on the raceway of a bearing. This will cause a minute periodic change in the coefficient of friction between the rolling element and the raceway and thus lead to self-excited vibration of the rolling element of bearing. Depending on the operating conditions, the self-excited vibration may produce a certain abnormal sound from the bearing, i.e., peculiar noise (peculiar noise at low temperature). To solve the above-mentioned problem, JP 11-270566 A discloses a grease composition where a particular base oil is contained at a specific ratio in order to lower the viscosity.
From the viewpoint of a recent tendency toward reduction in power consumption, electrical machinery and appliances and mechanical parts used in the automobiles as well as in various industries are required to have improved efficiency. Thus, various studies have been conducted to reduce the weight of parts and improve the structure. It also becomes important for the grease used together to show low torque performance across a wide temperature range from low temperatures up to high temperatures. The conventional step to improve the grease so as to satisfy the low torque performance was reduction of the kinematic viscosity of the employed base oil. For example, JP 2000-198993 A proposes a grease composition where a base oil comprises an ester oil with a kinematic viscosity at 40° C. of 10 mm2/s or more. However, when the kinematic viscosity of the base oil is reduced, the heat resistance of the base oil is so lowered at high temperatures that the seizure life will be unsatisfactory although the torque can be lowered.
In consideration of the operating environment of high temperatures, the grease selected is required to exhibit a long seizure life. To improve the seizure life under the circumstances of high temperatures, JP 1-259097 A proposes a grease composition comprising an alkyldiphenyl ether oil as an essential component and a particular thickener, for example. However, the above-mentioned grease cannot satisfy the low torque performance because of high kinematic viscosity of the base oil.
In the rolling bearings for motors, there is another special problem, i.e., rusting due to varnish. In this case, therefore, the grease is required to have a rust inhibiting effect. In terms of the grease, rather than ester type synthetic oils, mineral oils have been preferably selected for the base oil to improve the rust inhibiting effect on varnish (Suzuki, Tomaru, Yamamoto and Suzuki, “Cause and Prevention of Ball Bearing Rusting due to Insulating Varnish in Electric Motors”, NSK Bearing Journal, No. 643, 1982). However, the grease composition using mineral oil as the base oil cannot satisfy the low torque performance over a wide temperature range as mentioned above, especially at low temperatures. In addition, the mineral oil is inferior to the synthetic oil in the heat resistance, so that a long seizure life cannot be attained under the circumstances of high temperatures.
JP 2004-51912 A proposes a grease composition where a polyol ester is used as the rust inhibitor and the base oil contains a synthetic hydrocarbon oil in an amount of 10 mass % or more.
In this case, however, the rust inhibiting effect on varnish as desired in the rolling bearing for motor cannot be obtained.
As previously explained, the grease used in the rolling bearing for motors is required to prevent a peculiar noise from being generated at low temperatures, exhibit low torque performance over a wide temperature range, show a long seizure life under the circumstances of high temperature, and exhibit excellent rust inhibiting effect. Any of the above-mentioned grease compositions cannot satisfy all the performance.